Computer flash apparatus with series-connected control switches

ABSTRACT

A first and second thyristor each have an anode-cathode circuit connected in series with the flash tube. The required voltage rating corresponds to only 1/2 the voltage across the capacitor furnishing the energy for the flash. To assure ignition of both thyristors, the gate of each is connected through a resistor to a separate secondary winding of a transformer having a primary winding connected in the ignition circuit. To achieve rapid switching from the conductive to the non-conductive state, a resistor is connected in series with the cathode of each thyristor and the other terminal of each resistor is connected to the opposite terminal of the secondary winding connected to its gate. The resultant negative bias on the gate becomes effective at low thyristor currents to increase the switching speed to the non-conductive state.

United States Patent 1191 I I Mahlich et al. 4

[4 Nov. 11, 1975 [5 COMPUTER FLASH APPARATUS WITH SERIES-CONNECTEDCONTROL 1 SWITCHES [75] Inventors: Gotthard Mahlich, Kronberg; HeinzEngelstiitter, Bad Soden, both of Germany [73] Assignee: BraunAktiengesellschaft, Kronberg, Taunus, Germany 22 Filed: June 18, 1974[21 App]. No.: 480,501

52 us. or 315/241 P; 315/241 R 51 Int. (:1. HOSB 39/00 58 Field ofSearch 315/241 P [56] References Cited UNITED STATES PATENTS 3,809.95]5/1974 Vital et a1 315/241 P X 3.809.954 5/1974 Engelstzitternn 315/241P $814,985 6/1974 Pecher et a1. .1 315/241 P Primary E.raminerNathanKaufman Attorney, Agent, or Firm-Michael S. Striker 1 5 7 1 ABSTRACT Afirst and second thyristor each have an anodecathode circuit connectedin series with the flash tube. The required voltage rating correspondsto only /2 the voltage across the capacitor furnishing the energy forthe flash. To assure ignition of both thyristors, the gate of each isconnected through a resistor to a separate secondary winding of atransformer having a primary winding connected in the ignition circuit.To achieve rapid switching from the conductive to the nonconductivestate, a resistor is connected in series with the cathode of eachthyristor and the other terminal of each resistor is connected to theopposite terminal of the secondary winding connected to its gate. Theresultant negative bias on the gate becomes effective at low thyristorcurrents to increase the switching speed to the non-conductive state.

4 Claims, 1 Drawing Figure US. Patent Nov. 11, 1975 COMPUTER FLASHAPPARATUS WITH SERIES-CONNECTED CONTROL SWITCHES BACKGROUND OF THEINVENTION This invention relates to computer flash units. In particularit relates to such flash units when a flash tube is connected in serieswith electronic switches which interrupt the flash tube current. Theso-formed series circuit is connected in parallel with a capacitor whichfurnishes the energy required for the flash.

Circuits of this type are known wherein the anodecathode circuit of athyristor connected in series with a flash tube serves to interrupt thecurrent through the flash tube thereby terminating the flash.Specifically, the thyristor is ignited, that is switched to theconducting state, substantially simultaneously with the flash tube bythe application of an ignition signal generated in the ignition circuitto the gate of the thyristor. The thyristor is switched to thenon-conducting state at the end of the exposure time by the discharge ofa terminating capacitor which furnishes a current to the thyristor inopposition to the current furnished by the flash tube. This current ismaintained for a sufficiently long time that all charge carriers in thecathode-anode circuit of the thyristor are eliminated. When, therefore,current flow through the thyristor ceases, it is again in thenon-conductive state and the flash has been terminated.

Apparatus of this type has the disadvantage that, since the flash tuberequires a high voltage for ignition, a thyristor connected in seriestherewith must have a correspondingly high voltage rating, that is mustbe able to withstand a breakdown voltage exceeding the voltage acrossthe capacitor furnishing the energy for the flash. Further, in orderthat the capacitor which stores the energy for the terminating currentbe small, these thyristors are also required to have high reversecurrent ratings.

SUMMARY OF THE INVENTION It is an object of the present invention tofurnish an automatic flash apparatus having a series switch arrangementfor terminating the flash which does not have the above disadvantages,that is wherein the voltage rating of the switches (thyristors) can bedecreased to approximately half the voltage of the energy storingcapacitor and also having a lower power rating.

The present invention resides in a computer flash apparatus havingenergy storage means for storing the energy required for the flash. Itcomprises a series circuit including flash generating means responsiveto an ignition signal for furnishing said flash and a first and secondswitch each having a main conducting circuit connected in series withsaid flash generating means and having, respectively, a first and secondcontrol elec* trode, each of said switches switching from anonconductive to a conductive state in response to a control signal atsaid first and second control electrode respectively. It furthercomprises first and second connecting means for connecting said firstand second control electrode, respectively, to said ignition circuit insuch a manner that said ignition circuit furnishes said control signalsto said control electrodes substantially simultaneously with saidfurnishing of said ignition signal to said flash generating means.

In a preferred embodiment of the present invention the first and secondswitch are each a thyristor.

In a further preferred embodiment of the present invention, the firstand second connecting means are, respectively, a first and second.ignition resistor. Each of the ignition resistors serves to maintain thecontrol voltage at the control electrode sufficiently long to assureignition of the second thyristor to be switched when the switching ofboth does not take place exactly simultaneously. This decreases the timeduring which the second thyristor to be ignited has a high resistance,thereby decreasing the undesired power loss in said thyristor.

Since thyristors of high power ratings are extremely expensive, thesavings created by the use of two thyristors of lower rating more thencompensates for the additional circuit elements required for the presentinvention over those circuits utilizing only a single thyristor.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE shows a circuitdiagram of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of thepresent invention will now be described with reference to the drawing.

Shown in FIG. 1 is a storage means, namely a storage capacitor 1.Connected in parallel with storage capacitor 1 is the flash generatingmeans, namely a flash tube 2 connected in series with a first and secondswitch namely a thyristor 3 and 4. Ignition circuit means, which includea switch 5 which, subject to external activation, causes a pulse toappear across a transformer 6 which is coupled to the flash tube 2 andfurnishes an ignition pulse when switch 5 is closed. The ignitioncircuit further has a transformer 7 which has a first secondary winding71 and a second secondary winding 72. The first terminal of secondarywinding 71 is connected through a resistor 81 (one embodiment of a firstignition resistor) to the gate 31 of thyristor 3, while the firstterminal of secondary winding 72 is connected through the secondignition resistor 82, to the gate 41, of thyristor 4. An exposuremeasuring circuit which, as is well known, contains a photosensitiveelement which is responsive to the light reflected from the object beingphotographed furnishes a terminating signal when the so-received lighthas reached a predetermined light quantity. The terminating signal isapplied to an electronic switch 9. When switch 9 switches to aconducting state in response to this terminating signal, a previouslycharged terminating capacitor 10 is discharged. The polarity oncapacitor 10 is such that the current through thyristors 3 and 4resulting from the discharge of this capacitor is in a directionopposing the current from the flash tube 2. The capacitance of capacitor10 is such that this current is maintained sufficiently long so that allcharge carriers in the thyristors are eliminated. In other words, thecurrent through thyristors 3 and 4 becomes zero and these thyristors arethus effectively in the blocked state until such time as the controlvoltage is again applied to the gates. When thyristors 3 and 4 are inthe blocked state no current can flow through flash tube 2 and the flashis of course terminated. The speed of switching back to thenonconductive state can further be increased by the action of seriesresistors 110 and 111 each connected to the cathode of one of thethyristors in such a manner that the current flow therethrough creates anegative bias on the gate of the corresponding thyristor. Of course anyincrease in the switching speed results in lesser loading of thethyristors and is thus very desirable.

It is seen that the circuit of the present invention serves to terminatethe flash reliably while requiring thyristors having decreased ratingsfrom the thyristors normally employed thereby effecting the substantialdecrease in the cost of the equipment.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgeadapt it for various applications without omitting features that, fromthe standpoint of prior art fairly constitute essential characteristicsof the generic or specific aspects of this invention and, therefore,such adaptations should and are intended to be comprehended within themeaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

1. In electronic flash apparatus having storage means for storing theenergy required for said flash, said storage means having storage outputtenninals furnishing a predetermined voltage when said storage means ischarged, in combination, a series circuit connected across said outputterminals, said series circuit comprising flash generating means forfurnishing said flash in response to an ignition signal, and a first andsecond switch each having a main conducting circuit connected in serieswith said flash generating means and having, respectively, a first andsecond control electrode, each of said main conducting circuitsswitching from a non-conductive to a conductive state in response to acontrol signal at the respective control electrode; ignition circuitmeans coupled to said flash generating means for furnishing saidignition signal upon external activation; first and second connectingmeans for connecting said ignition circuit to said first and sec- 0ndcontrol electrodes in such a manner that said control electrodes receivesaid control signals substantially simultaneously with the furnishing ofsaid ignition signal to said flash generating means thereby switchingsaid main conducting circuits to the conducting state;

and terminating circuit means connected to said main conductingcircuits, for blocking the current flow therethrough thereby terminatingsaid flash at the end of the exposure time.

2. Apparatus as set forth in claim 1, wherein said storage meanscomprise a capacitor; wherein said series circuit is connected inparallel with said capacitor; and wherein said first and second switchare, respectively, a first and second thyristor, each of said thyristorshaving a gate constituting said control electrode and an anode-cathodecircuit constituting said main conducting circuit.

3. Apparatus as set forth in claim 2, wherein said ignition circuitmeans comprise a transformer having a first and second secondarywinding; and wherein said first connecting means comprise a firstignition resistor connected between said first secondary winding andsaid gate of said thyristor and said second connecting means comprise asecond ignition resistor connected between said second secondary windingand said gate of said second thyristor.

4. Apparatus as set forth in claim 3, further comprising a first andsecond series resistor connected in said series circuit, each having afirst terminal connected to the cathode of a corresponding one of saidthyristors, and a second terminal; wherein said first secondary windingis connected between said first ignition resistor and said secondterminal of said first series resistor and said second secondary windingis connected between said second ignition resistor and said secondterminal of said second series resistor; and wherein said terminatingmeans comprise a normally charged capacitor, means for initiating thedischarge of said capacitor at the end of said exposure time, and meansfor connecting said capacitor to said switches in such a manner thatdischarge of said capacitor blocks said current flow through said mainconductucting circuit thereby switching said switches to thenon-conducting state and terminating said flash.

1. In electronic flash apparatus having storage means for storing theenergy required for said flash, said storage means having storage outputterminals furnishing a predetermined voltage when said storage means ischarged, in combination, a series circuit connected across said outputterminals, said series circuit comprising flash generating means forfurnishing said flash in response to an ignition signal, and a first andsecond switch each having a main conducting circuit connected in serieswith said flash generating means and having, respectively, a first andsecond control electrode, each of said main conducting circuitsswitching from a non-conductive to a conductive state in response to acontrol signal at the respective control electrode; ignition circuitmeans coupled to said flash generating means for furnishing saidignition signal upon external activation; first and second connectingmeans for connecting said ignition circuit to said first and secondcontrol electrodes in such a manner that said control electrodes receivesaid control signals substantially simultaneously with the furnishing ofsaid ignition signal to said flash generating means thereby switchingsaid main conducting circuits to the conducting state; and terminatingcircuit means connected to said main conducting circuits, for blockingthe current flow therethrough thereby terminating said flash at the endof the exposure time.
 2. Apparatus as set forth in claim 1, wherein saidstorage means comprise a capacitor; wherein said series circuit isconnected in parallel with said capacitor; and wherein said first andsecond switch are, respectively, a first and second thyristor, each ofsaid thyristors having a gate constituting said control electrode and ananode-cathode circuit constituting said main conducting circuit. 3.Apparatus as set forth in claim 2, wherein said ignition circuit meanscomprise a transformer having a first and second secondary winding; andwherein said first connecting means comprise a first ignition resistorconnected between said first secondary winding and said gate of saidthyristor and said second connecting means comprise a second ignitionresistor connected between said second secondary winding and said gateof said second thyristor.
 4. Apparatus as set forth in claim 3, furthercomprising a first and second series resistor connected in said seriescircuit, each having a first terminal connected to the cathode of acorresponding one of said thYristors, and a second terminal; whereinsaid first secondary winding is connected between said first ignitionresistor and said second terminal of said first series resistor and saidsecond secondary winding is connected between said second ignitionresistor and said second terminal of said second series resistor; andwherein said terminating means comprise a normally charged capacitor,means for initiating the discharge of said capacitor at the end of saidexposure time, and means for connecting said capacitor to said switchesin such a manner that discharge of said capacitor blocks said currentflow through said main conductucting circuit thereby switching saidswitches to the non-conducting state and terminating said flash.